1. Field of the Invention
This application relates to purification of eukaryotic pathogens and parasites, particularly, Plasmodium sporozoite-stage parasites. More particularly it relates to substantially pure parasites and methods of preparing and using them. The application also relates to vaccine and pharmaceutical compositions of purified sporozoite stage Plasmodium parasites, both attenuated and non-attenuated, and methods of using the compositions in vaccines and other preparations to prevent malaria and other diseases, treat diseases, and as a means to infect volunteers in the testing of malaria vaccines and drugs.
2. Background Art
Malaria is a disease that is estimated to affect 300-500 million people and kills 1-3 million individuals annually. It also has an enormous economic impact on people in the developing world, especially those in sub-Saharan Africa. Plasmodium falciparum accounts for the majority of deaths from malaria in the world. The World Tourist Organization reported that of the nearly 700 million international tourist arrivals recorded worldwide in 2000, approximately 9 million were to West, Central or East Africa, 37 million were to South-East Asia, 6 million to South Asia and 10 million to Oceania. It is estimated that more than 30,000 travelers from North America, Europe, and Japan contract malaria per year. For more than 100 years during every military campaign conducted where malaria was transmitted, U.S. forces have had more casualties from malaria than from hostile fire. An estimated 12,000,000 person days were lost during World War II and 1.2 million during the Vietnam conflict due to malaria.
Transmission of the Plasmodium parasite occurs through the bite and feeding of infected female Anopheles mosquitoes which are active from dusk to dawn. Plasmodium, at the sporozoite stage of development, migrate from the bite site to the liver, primarily via the blood stream, where they multiply within hepatocytes, producing, in the case of P. falciparum, about 10,000-40,000 progeny per infected cell. These liver stage parasites express some proteins which are not expressed at the sporozoite stage. At this stage, parasites re-enter the blood stream as merozoites, expressing some proteins which are different from those expressed during the sporozoite and early hepatic stages, and invade erythrocytes, where additional multiplication increases parasite numbers by approximately 10 to 20 fold every 48 hours. Unlike the five to ten day development in the liver, which does not induce any symptoms or signs of malaria, untreated blood stage infection causes hemolysis, shaking chills, high fevers, and prostration. In the case of P. falciparum, the most dangerous of the four major species of Plasmodium that cause human disease (P. vivax, P. malariae, and P. ovale [P. knowlesi can also cause human disease]), the disease is complicated by disruption of microcirculatory blood flow and metabolic changes in vital organs such as the brain, kidneys and lungs, frequently leading to death if not urgently treated.
An effective vaccine against P. falciparum malaria remains one of the great challenges of medicine. Despite over one hundred years of effort, hundreds of millions of dollars in research, lifelong sacrifice from dedicated physicians and scientists, and many promising experimental vaccines, there is no marketed vaccine to alleviate one of the great infectious scourges of humanity.
A generation ago, public health initiatives employing chloroquine, DDT and vector control programs seemed poised to consign falciparum malaria to insignificance as a worldwide menace. The lack of an effective vaccine complicated these efforts, but sustainable control seemed imminent.
The promise of impending success was short-lived and the reasons for failure were multi-factorial. The parasites grew increasingly resistant to highly effective and affordable anti-malarial medications, vector control measures lapsed, and trans-migration, war and economic disruption became increasingly more common in endemic areas of the developing world. As a result, P. falciparum malaria resurged, annually placing at least 2.5 billion humans at risk, causing 300-900 million infections, and killing 1-3 million people. Of the many social, economic, environmental and political problems that afflict the developing world, P. falciparum malaria is increasingly seen as both a root cause and cruel result of these inequities, and is a singular impediment to solving these complex problems. Controlling P. falciparum malaria in the developing world may not be possible without an effective vaccine. In practice, given social, political and economic realities, we believe that a vaccine may be an essential component of a sustainable control program, and will be required for a global eradication campaign.
During the last 25 years most research effort has been spent on identifying the antigenic subunits of the parasite which confer immunity—unfortunately, with less than satisfying results. This effort and the attendant difficulties in developing a suitable vaccine have been described (Nussenzweig V., F. and R. S. Nussenzweig, Adv. Immunol., (1989) 45: 283-334; Hoffman S. L. et al. In: Hoffman S. L., ed. Malaria Vaccine Development: A Multi-Immune Response Approach (1996) Washington, D.C.: ASM Press, pp. 35-75; Hoffman S. L. and L. H. Miller, In: Hoffman S. L., ed. Malaria Vaccine Development: a Multi-Immune Response Approach (1996) Washington, D.C.: ASM Press, pp. 1-13; Epstein, J. E. et al, Curr. Opin. Mol. Ther. (2007) 9:12-24; Richie, T. L. & A. Saul, Nature, (2002) 415:694-701).
There are continuing efforts to produce subunit malaria vaccines. Typical of such attempts, Paoletti et al. (U.S. Pat. No. 5,766,597, issued Jun. 16, 1998) disclose a recombinant poxvirus containing DNA from Plasmodium coding for one or more circumsporozoite proteins, including an embodiment termed NYVAC-Pf7, possibly useful as a potential malaria vaccine. Subsequent testing of this construct proved to be disappointing (Ockenhouse, C. F. et al. J. Infect. Dis. (1998) 177: 1664-73).
Similarly, another candidate subunit circumsporozoite vaccine was proposed and identified as RTS, S/AS02A (Stoute J. A. et al. J. Infect. Dis. (1998) 178: 1139-44). The results of the first Phase 2b field trial of this vaccine in one-four year old children in Mozambique was reported (Alonso, P. L. et al. Lancet (2004) 364:1411-1420; Alonso, P. L. et al. Lancet (2005) 366:2012-2018; Epstein, J. E. et al, Supra; Richie, T. L., F. & A. Saul, Supra), as were the results of other Phase 2b field trials in infants (Aponte, J. J. et al. (2007) The Lancet 370:1543-1551; Bejon, P. et al (2008) NEJM 359:2521-32; Abdullah, S et al (2008) NEJM 359:2533-44. The vaccine has demonstrated modest protective efficacy.
On the other hand, the demonstration of the effectiveness of whole parasite, radiation attenuated sporozoites (delivered to human hosts by mosquito exposure and to animal hosts by intravenous (i.v.) inoculation) in conferring high levels of protective immunity when recipients are subsequently challenged with pathogenic parasites (most importantly attenuated Plasmodium falciparum to human hosts) was an early milestone in the quest for a suitable vaccine (Hoffman S. L. et al., J. Infect. Dis. (2002) 185: 1155-64). Eventually, this led to efforts to explore the technical hurdles which present themselves in transforming these earlier observations into a practical vaccine approach comprising aseptic attenuated sporozoites (Luke, T. C. & S. L. Hoffman, J. ExP. Biol., (2003) 206:3803-3808; Hoffman, S. L., and T. C. Luke U.S. Pat. No. 7,229,627 and U.S. Publication 2005/0208078). It has also led to increased interest generally in the utility of vaccines utilizing attenuated sporozoites (Menard, R., Nature 2005) 433:113-114; Waters, A. P. et al. Science. (2005) 307:528-530; Wykes, M. F. & M. F. Good, Int. J. Parasitol. (2007) 37:705-712; Renia, L. et al, Expert Rev. Vaccines, (2006) 5:473-481; Epstein, J. E. et al, Supra).
Other modes of attenuation have also been demonstrated. For example, It was shown that attenuated sporozoites resulting from gene alteration of Plasmodium berghei protects mice against P. berghei malaria (Kappe et al. U.S. Pat. No. 7,122,179; Mueller et al. Nature (2005); Mueller, et al. PNAS (2005); van Dijk et al. PNAS (2005) 102:12194-12199); Waters, U.S. Pat. No. 7,550,138; Labeled et al. Infect. And Immun. (2007). Recently genetic attenuation of the sporozoites of P. falciparum has be disclosed (van Schaijk et al. PLoS ONE (2008) 3:e3549); VanBuskirk et al. PNAS.
Similarly, chemical attenuation of Plasmodium has be described (Purcell et al. Infect. Immun. (2008)76:1193-99; Purcell et al Vaccine (2008) 26:4880-84).
Others have also described methods of culturing unpurified preparations of sporozoites and inducing parasite differentiation to axenic liver stages (Kappe et al. US Pub. 2005/0233435).
The studies discussed above set forth certain limitations. For example, while sporozoites delivered to human hosts by the bite of a mosquito generate an effective immune response against malaria, such a method of delivery is clearly not a practical method for vaccinating a population in need of protection against malaria. Additional studies in mice, referred to above, have suggested that the delivery of attenuated sporozoites to mice intravenously are also effective, whereas other means of delivery (e.g., intramuscular) in comparison, are not. An intravenous delivery method, however, is also not practical if a malarial vaccine is to be delivered to numerous individuals (including children and the eldery). Intravenous delivery has increased risks, increased costs, and patients are far less likely to agree to be vaccinated using such a method. Thus, there was a need in the art to provide an effective malarial vaccine that provides protective immunity, where the vaccine can be administered by a variety of methods.
With regard to considerations for human vaccines, the purity of the immunogen and the presence or absence of non-specific, attendant material which may be immunogenic or toxic are further essential issues which have not previously been resolved. Isolated attenuated sporozoite preparations, as used in studies discussed above, contain contaminating and other attendant material. There was a need in the art to develop sporozoite-based vaccines, particularly in humans, that employ aseptically prepared, sterile, purified preparations of sporozoites for use in vaccine compositions. Such aspetically prepared, purified preparations of sporozoites may also be more effective than non-purified preparations when such preparations are administered by non-intravenous delivery methods. (e.g., intramuscular, intraperitoneal, intradermal, epidermal, mucosal, submucosal, cutaneous, or subcutaneous).